add local tox targets for pep8, py3 and py27
add local tox targets for pep8, py3 and py27

This patch adds local tox targets for pep8 and python 3 as per [1]
that will install dependencies in editable mode.
To run them use the pep8-dev and py3-dev targets respectively.

[1] https://etherpad.openstack.org/p/neutron-sibling-setup

Change-Id: I5a43dc795fe2b02a5e9a17525f3f9c2c1ff42779

openstack/networking-sfc,openstack/networking-sfc